This invention relates to electrostatographic printing machines, and more particularly to a liquid ink development (LID) electrostatographic printing machine having a fluid film thickness control apparatus for controlling the development nip gap, and hence thickness of liquid developer material or development fluid in the development nip, of such a machine.
A typical electrostatographic printing machine employs a photoconductive member that is sensitized by charging to a substantially uniform potential. The charged portion of the photoconductive member is exposed to the light image of a document. Exposure of the charged photoconductive member selectively dissipates the charge to record an electrostatic latent image. The electrostatic latent image corresponds to the informational areas of the document. The electrostatic latent image recorded on the photoconductive member is developed by contact with a developer material or development fluid. The developer material or development fluid can be a dry material comprising carrier granules having adhering toner particles. The latent image attracts the toner particles from the carrier granules to form a toner powder image on the photoconductive surface. The toner powder image is then transferred and permanently fused to a copy sheet.
An electrostatic latent image also may be developed with a liquid developer material or development fluid. In a liquid development system, the photoconductive member is contacted with an insulating liquid carrier having dispersed finely divided marking particles. The electrical field associated with the electrostatic latent image attracts the marking particles to the photoconductive member to form a visible image.
Liquid developing imaging processes utilize a liquid developer typically having about 2 percent by weight of fine solid particulate toner material dispersed in a liquid carrier. The liquid carrier is typically a hydrocarbon. In the developing process, the image is transferred to a receiver which may be an intermediate belt. The image on the photoconductive member contains about 12 weight percent of particulate toner in liquid hydrocarbon carrier. In order to improve the quality of transfer of a developed image to a receiver, the percent solids in the liquid should be increased to about 25 percent by weight. Increase in percent solids may be achieved by removing excess hydrocarbon liquid. However, excess hydrocarbon liquid must be removed in a manner that results in minimum degradation of the toner image.
Prior art liquid ink development systems operate such that a photoconductor surface rotates through a developer bath to make contact with the toner. In these systems, toner particles are attracted to a latent electrostatic image on the photoconductor surface. The motion of the toner particles in an imagewise electric field is generally called electrophoresis and is well known in the art. However, a liquid carrier also wets the photoconductor surface. It is very difficult to transfer a toner image to paper without either first removing the liquid carrier from the photoconductor surface or using the liquid carrier to enable transfer to the paper and subsequently removing the liquid carrier from the paper. In both cases, the liquid carrier must be removed by processes that must include evaporation of the liquid carrier into the air, which causes airborne pollution.
U.S. Pat. No. 4,707,112, to Hartmann, Nov. 17, 1987, relates to an apparatus for developing an electrostatic latent image. The apparatus includes means for furnishing liquid developer material or development fluid to the image in a development zone and means for dispersing the particles substantially uniformly in the liquid carrier of the liquid developer material or development fluid at the entrance to the development zone so as to deflocculate marking particles. The dispersing means may comprise means for generating a pulsed electrical field in the developer material or development fluid at the entrance to the development zone to induce movement of the marking particles and the liquid carrier. The generating means includes an electrode positioned at the entrance to the development zone and means for applying a pulsed voltage to the electrode to generate a pulsed electrical field in the developer material or development fluid.
U.S. Pat. No. 5,157,443 to Anderson et al, Oct. 20, 1992 relates to liquid development of latent images produced on a movable image retention belt for high speed reproducing machines by using a moving belt applicator to define a development zone having a uniform gap with an extended length. This disclosure requires the gap to be set and controlled by the accuracy of the parts involved and their mounting arrangements.
U.S. Pat. No. 5,619,313 to Domoto et al, Apr. 8, 1997, relates to a method and apparatus for simultaneously developing and transferring a liquid toner image by moving a latently imaged photoreceptor and a biased intermediate transfer member into a process nip forming a relationship. The method further includes the step of introducing charged liquid toner into the process nip, such that liquid toner sandwiched within the nip simultaneously develops image portions of the latent image onto the intermediate transfer member, and background portions of the latent image onto the charged bearing surface of the photoreceptor.
In any LID machine as described above, the process speed or rate of image development is a function of the electrostatic field within the development nip, of the toner particle size and electric charge, and of the viscosity of the carrier fluid or liquid developer material or development fluid. Higher process speeds and higher developer material or development fluid viscosities demand smaller development nip gaps and longer development nips or zones. Effective control of such nip gaps by attempting to accurately position external parts has been found to be relatively difficult and expensive due in part to the mechanical tolerances involved.
There is therefore a need for a comparatively easy and less costly approach to accurately controlling process speeds by controlling the development nip gap, and hence thickness of liquid developer material or development fluid in the development nip, of such a machine
In accordance with the present invention, there is provided apparatus for controlling a thickness (h) of a film of development fluid within the development nip of a liquid ink development (LID) machine. The apparatus includes a first moveable member having a first velocity (V1) and a path of movement defining a radius of curvature (R) for forming a long development nip. It also includes a second moveable member mounted partially about the first moveable member for forming the long development nip, and having a second velocity (V2) as well as a tension (T). The apparatus further includes a mechanism for introducing into the long development nip liquid development fluid having a viscosity (xcexc), and a programmed controller connected to the first moveable member and the second moveable member, for controlling the first velocity (V1), the second velocity (V2), and the tension (T) so that h=K1R{K2xcexc(V1+V2)]/T}⅔, where K1 and K2 are each a constant.
Other features of the present invention will become apparent from the following drawings and description.